Clinical signs and physical function in neck and upper extremities among elderly female computer users: the NEW study

The aim of the study was to present the prevalence of clinical signs and symptoms among female computer users above 45 years, both in a group with self-reported neck/shoulder trouble (NS cases) and in a group without such trouble (NS controls). The hypothesis was that computer users with self-reported neck/shoulder trouble have more clinical findings than those not reporting trouble, and that a corresponding pattern holds true for physical function. In total 42 and 61 questionnaire-defined NS cases and NS controls participated and went through a clinical examination of the neck and upper extremities and five physical function tests: maximal voluntary contraction (MVC) of shoulder elevation, abduction, and handgrip, as well as endurance at 30% MVC shoulder elevation and a physical performance test. Based on clinical signs and symptoms, trapezius myalgia (38%), tension neck syndrome (17%) and cervicalgia (17%) were the most frequent diagnoses among NS cases, and were significantly more frequent among NS cases than NS controls. A total of 60% of the subjects with reported trouble had one or several of the diagnoses located in the neck/shoulder. Physical function of the shoulder was lower in subjects with self-reported trouble as well as in the subgroup of NS cases with clinical diagnoses. In conclusion, the present clinical diagnoses and physical function tests differed between NS cases and NS controls, and are therefore recommended to be included as quantitative objective measures in assessing musculoskeletal health. Physical function tests should be further developed in order to be able to detect pre-stages of work-related disorders for preventive strategies.     

Comparison of the electromyographic activity in the upper trapezius and biceps brachii muscle in subjects with muscular disorders: a pilot study

The aim of the present study was to investigate the extent to which work-related muscular disorders of the upper trapezius affect the activity of other pain-free muscles, in particular in the biceps brachii. Two groups of female subjects (age >43 years) participated in the study: seven affected subjects with self-reported disorders in the shoulder/neck region (cases) and nine healthy subjects (control group). Multi-channel electromyography (EMG) and force were recorded during maximum voluntary contractions (MVC) and during 6 min sustained contractions (at 30% MVC) of the upper trapezius and biceps brachii on the dominant side. From the EMG signals, the root mean square (RMS), median frequency (MDF) and single motor unit (MU) conduction velocity (CV) were estimated. From the force signal, the coefficient of variation was calculated. All data are presented as mean values and standard deviation. Differences between the cases and controls were found in the MVC force of the upper trapezius, which was lower in cases [253 (70) N] than in controls [357 (75) N], while the coefficient of variation of force during the sustained contraction was increased [cases 5.5 (2.2); controls 4.1 (1.9)]. The RMS (normalized to the RMS at MVC) during the 6 min sustained contractions was significantly lower in the cases than in the controls for both the upper trapezius and the biceps brachii. A tendency towards a smaller increase in the RMS with fatigue was only found in the trapezius muscle [slope: cases 6.5 (14.1) %/min, controls 10.2 (12.9) %/min]. No differences were found between the two subject groups with respect to the MDF and single MU CV in both muscles. While the lower RMS for the trapezius muscles of the cases may reflect changes at the local level, as well as in motor control, the lower biceps activity indicates a change in the central control strategies of the primarily unaffected muscle. Indications for a changed fatigability of the muscle were only found in the trapezius.     

Shoulder and forearm oxygenation and myoelectric activity in patients with work-related muscle pain and healthy subjects

We tested hypotheses of (a) reduced oxygen usage, oxygen recovery, blood flow and oxygen consumption; and (b) increased muscle activity for patients diagnosed with work-related muscle pain (WRMP) in comparison to healthy controls. Oxygenation was measured with near infrared spectroscopy (NIRS), and muscle activity with EMG for the extensor carpi radialis (ECR) and trapezius descendens (TD) muscles. Eighteen patients with diffuse neck–shoulder–arm pain and 17 controls (matched in age and sex) were equipped with NIRS and EMG probes. After determining an individual’s maximum voluntary contraction (MVC) force, short-term (20 s) isometric contractions for the ECR and TD of 10, 30, 50 and 70 % MVC generated ?StO₂ and StO₂% recovery (Rslope) from NIRS, and RMS%_max from EMG signals. In addition, upper arm venous (VO) and arterial (AO) occlusions generated slopes of total hemoglobin (HbTslope) and deoxyhemoglobin (HHbslope) for the resting ECR as surrogates of blood flow and oxygen consumption, respectively. Mixed model analyses, t tests, and Mann–Whitney test were used to assess differences between groups. There was no significant difference in MVC between groups for either muscle. Also, ?StO₂%, Rslope for either muscle, and ECR-HbTslope were not different between groups, thus our hypotheses of reduced oxygen use, recovery, and blood flow for patients were not confirmed. However, patients had a significantly lower ECR-HHbslope confirming our hypothesis of reduced consumption. Further, there was no difference in RMS%_max during contractions meaning that the hypothesis of increased activity for patients was not confirmed. When taking into account the number of NIRS variables studied, differences we found between our patient group and healthy controls (i.e., in forearm oxygen consumption and shoulder oxygen saturation level) may be considered modest. Overall our findings may have been impacted by the fact that our patients and controls were similar in muscle strength, which is in contrast to previous studies.     

Electrical and mechanical response of finger flexor muscles during voluntary isometric contractions in elite rock-climbers

To determine the differences between rock-climbers and controls in finger flexor (FF) motor units (MUs) features and activation strategy, eleven climbers and ten controls volunteered for the study. After maximal voluntary contraction (MVC) assessment, five levels of isometric contractions at 20, 40, 60, 80 and 100% MVC were performed. During contractions, electromyogram (EMG) and mechanomyogram (MMG) were recorded, from which the root mean square (RMS) and mean frequency (MF) were calculated. Climbers showed significantly higher MVC. EMG RMS was statistically higher in climbers than in controls from 60 to 100% MVC. In climbers MMG RMS increased up to 80% MVC, whereas in controls it increased only up to 60% MVC. MMG MF was higher in climbers than in controls from 60 to 100% MVC (P < 0.05). EMG–MMG combined analysis revealed significant differences in MU activation strategy between the two groups. The results are compatible with a shift of climbers’ muscles toward faster MUs.     

Performance and muscle activity during computer mouse tasks in young and elderly adults

The influence of age on performance and muscle activity was studied during computer mouse tasks designed to induce high demands on motor control. Eight young (mean age 25 years) and nine elderly (mean age 63 years) women participated. When the speed was self-determined, the elderly subjects performed 13%–18% slower than did the young. When speed was predefined, the error rate was higher in the elderly subjects than in the young ones (medium precision 7.8% compared to 2.5% , high precision 16.5% compared to 7.9%, respectively). The highest error rate was found for double-clicking (32.9% compared to 13.5%, respectively). The reduced performance in the elderly subjects was hypothesised to be a combined effect of deteriorated proprioception, increased motor unit size, and changes in the central nervous system. Electrical activity (EMG) was recorded from the forearm, shoulder and neck muscles. Higher levels of EMG activity were found in the elderly compared to the young. A likely explanation is that the impaired motor control necessitated an increased muscle activity. The highest levels of EMG activity and lack of EMG gaps were found for the forearm extensor muscles, especially the extensor digitorum muscle (mean EMG activity 10.4% compared to 8.1% of maximal electrical activity, EMG_max) whereas lower EMG activity levels were found for the shoulder region (e.g. right trapezius muscle mean EMG 2.8% compared to 1.1% EMG_max, respectively). The latter was possibly due to a relieving effect of the forearm support. Differences in muscle activity among the tasks were found, however they were minor for the shoulder and neck muscles. Consideration of the demands on motor control when designing user interfaces is recommended, to the benefit of both the young and the elderly. Electronic Publication     

Normalization of electromyogram in the neck-shoulder region

Summary Linear and curvilinear electromyogram (EMG) normalization methods were compared among ten healthy men during a simulated work cycle demanding attention and static holding of the arm (Solitaire test). Maximal voluntary contractions (MVC) and gradually increasing contractions up to 70% of MVC were used for normalization in different arm postures. The test contractions studied included inward and outward rotations, abduction, shoulder elevation, and flexion in different arm positions. The shoulder load moment was calculated for the flexion tests using a simple two-dimensional model. The effect of arm posture on the EMG versus shoulder load moment relationship was studied on the following muscles: supraspinatus, infraspinatus, trapezius (three parts), deltoid (two parts) and pectoralis major. All muscles participated in the MVC tests performed, and its was not possible to suggest a single recommended test for each muscle. Differences in normalized EMG median values ranging up to 30% of MVC were found between linear and curvilinear normalization methods. Short-term repeatability of normalization based on a contraction with gradually increasing force was good. Arm posture affected the relationships between shoulder load moment and EMG activity of all muscles studied. Arm posture did not, however, have a significant effect on the estimated amplitude probability distribution functions during the simulated work task. Therefore, at least for the tasks studied, the principle of normalizing in the middle position of the range of movement was deemed acceptable.     

Shoulder muscle strength and fatigability in patients with frozen shoulder syndrome: the effect of 4-week individualized rehabilitation

The effect of 4-week individualized rehabilitation on shoulder muscle strength and fatigability was evaluated in 10 patients with frozen shoulder syndrome (FSS) in comparison of 10 age- and gender-matched healthy control subjects. Isometric maximal voluntary contraction (MVC) force of the shoulder flexors was measured by hand-held dynamometer. Isometric endurance of the shoulder muscles was characterized by endurance time and net impulse (NI) assessed during weight (30% MVC) holding in hand until exhaustion. Fatigability of deltoideus, infraspinatus and trapezius muscles during isometric endurance test was assessed by electromyogram (EMG) power spectrum median frequency (MF) slope per minute. Rehabilitation in patients with FSS consisted of exercise therapy in swimming pool and gymnasium, electrical therapy and massage. Before rehabilitation, patients with FSS had less (p < 0.05) isometric MVC force and NI during endurance test compared to the control. MF slope in patients with FSS for involved extremity was higher (p <0.05) for the deltoideus muscle and less for the infraspinatus muscle before rehabilitation compared to the controls. Shoulder pain was decreased (p < 0.05) and isometric MVC force and NI in endurance test in patients for involved extremity were increased after rehabilitation. No significant changes in endurance time and MF slope for infraspinatus and trapezius muscles in patients for involved extremity were observed after rehabilitation, whereas MF slope for deltoideus muscle was increased. It was concluded that in patients with FSS, 4-week rehabilitation decreased shoulder pain and improved MVC force of the shoulder flexors and isometric working capacity of these muscles during endurance test.     

Electromyogram and mechanomyogram changes in fresh and fatigued muscle during sustained contraction in men

In surface electromyogram (EMG) and mechanomyogram (MMG) the electrical and mechanical activities of recruited motor units (MU) are summated. Muscle fatigue influences the electrical and mechanical properties of the active MU. The aim of this study was to evaluate fatigue-induced changes in the electrical and mechanical properties of MU after a short recovery period, using an analysis of force, surface EMG and MMG. In seven subjects the EMG and MMG were recorded from the biceps brachii muscle during sustained isometric effort at 80% of the maximal voluntary contraction (MVC), before (test 1) and 10 min after (test 2) a fatiguing exercise. From the time and frequency domain analysis of the signals, the root mean square (rms) and the mean frequency (f¯) of the power spectrum were calculated. The results were that the mean MVC was 412 (SEM 90) N and 304 (SEM 85) N in fresh and fatigued muscle, respectively; during tests 1 and 2 the mean EMG rms increased from 0.403 (SEM 0.07) mV to 0.566 (SEM 0.09) mV and from 0.476 (SEM 0.07) mV to 0.63 (SEM 0.09) mV, respectively; during test 1 the mean MMG rms decreased from 9.4 (SEM 0.8) mV to 5.7 (SEM 0.9) mV; in contrast, during test 2 constantly lower values were observed throughout contraction; during tests 1 and 2 the EMG f¯ declined from 122 (SEM 7) Hz to 74 (SEM 7) Hz and from 106 (SEM 8) Hz to 60 (SEM 7) Hz, respectively; during test 1 the MMG f¯ increased in the first 6 s from 19.3 (SEM 1.4) Hz to 23.9 (SEM 2.9) Hz, falling to 13.9 (SEM 1.3) Hz at the end of contraction; in contrast, during test 2 the MMG f¯ declined continuously from 18.7 (SEM 1) Hz to 12.4 (SEM 0.8) Hz. The lower MVC after the fatiguing exercise and the changes in the EMG parameters confirmed that 10 min after the fatiguing exercise, the mechanical and electrical activities of MU were altered. In addition, the MMG results suggested that after a 10-min recovery, some highly fatigable MU might not be recruitable.     

Experimental pain leads to reorganisation of trapezius electromyography during computer work with active and passive pauses

The aim of this laboratory study was to investigate acute effects of experimental muscle pain on spatial electromyographic (EMG) activity of the trapezius muscle during computer work with active and passive pauses. Twelve healthy male subjects performed four sessions of computer work for 2 min in one day, with passive (relax) and active (30% maximum voluntary contraction of shoulder elevation) pauses given every 40 s without and with presence of experimental pain. Surface EMG signals were recorded from four parts of the trapezius. The centroid of exposure variation analysis along the time axis was lower during computer work with active pauses when compared with passive one in all muscle parts (P < 0.05). In presence of experimental pain, EMG amplitude increased in transverse and ascending parts and relative rest time decreased in ascending part. The results of this study showed a more variable trapezius activity pattern and increased activity with active compared with passive pauses, a lowered trapezius rest with presence of experimental pain, and increased activity in the transverse and ascending parts of trapezius due to experimental pain during computer work. Acute pain led to muscle activation pattern during computer work considered to increase the risk of developing work-related musculoskeletal disorders.     

Periodic increases in force during sustained contraction reduce fatigue and facilitate spatial redistribution of trapezius muscle activity

This study compared fatigue and the spatial distribution of upper trapezius electromyographic (EMG) amplitude during a 6-min constant force shoulder elevation task at 20% of the maximal voluntary contraction force (MVC) (constant force) and during the same task interrupted by brief (2 s) periodic increases in force to 25% MVC every 30 s (variable force). Surface EMG signals were recorded with a 13 × 5 grid of electrodes from the upper trapezius muscle of nine healthy subjects. The centroid (center of activity) of the EMG root mean square map was computed to assess changes over time in the spatial distribution of EMG amplitude. MVC force decreased by (mean ± SD) 9.0 ± 3.9% after the constant force task (P < 0.05) but was unchanged following the variable force contraction. The centroid of EMG amplitude shifted in the cranial direction across the duration of the variable force contraction (P < 0.05) but not during the constant force contraction (shift of 2.9 ± 2.3 mm and 1.4 ± 1.1 mm, respectively). The results demonstrate that periodic increases in force during a sustained contraction enhance the modifications in spatial distribution of upper trapezius EMG amplitude and reduce fatigue compared to a constant force contraction performed at a lower average load. The change in spatial distribution of EMG amplitude over time during a sustained contraction may reflect a mechanism to counteract fatigue during prolonged muscle activity.     

Using risk factors, myoelectric signal, and finger tremor to distinguish computer users with and without musculoskeletal symptoms

Most of previous studies use questionnaire to assess risk factors for cumulative trauma disorders of the upper extremity (CTDUE) for computer workers. Few studies combine both physical examination and questionnaire to assess musculoskeletal symptoms. Fifteen symptomatic and 15 non-symptomatic computer users were recruited. Both of them were asked to perform a repetitive tapping task (200 taps/min) as the fatigue task. Tremor of the index finger and surface electromyography (EMG) of the flexor digitorum superficial (FDS) were collected prior and after the tapping task. Muscle strength and range of motion for right wrist were collected before the tapping task. All subjects were asked to fill out the questionnaire about risk factors of CTDUE. Female users in the symptomatic group had weaker wrist extensor strength than those in the non-symptomatic group (P < 0.05). After performing the tapping task, FDS strength and median frequency of the FDS EMG at 25%, and 100% maximal voluntary contraction (MVC) for the symptomatic group decreased (P < 0.05). However, no significant differences were found in strength and median frequency of the FDS EMG between prior and after tapping task in the non-symptomatic group. There was no significant difference for root mean square of the finger tremor between and within groups. Regression analysis revealed that median frequency of the FDS EMG at 25% MVC, age, total time spent on computer, and mouse position were better factors to classify computer users into the symptomatic group compared to other factors obtained from questionnaire and physical exam. Besides, symptomatic computer users seem to have longer experience of computer use than non-symptomatic users. After the fatigue task, the symptomatic users decreased their muscle strength to a larger extent than the non-symptomatic users.     

Time and frequency domain analysis of electromyogram and sound myogram in the elderly

The aim of this work was to evaluate the influence of the ageing process on the time and frequency domain properties of the surface electrical and mechanical activity of muscle. In 20 healthy elderly subjects (10 men and 10 women, age range 65–78 years) and in 20 young controls, during isometric contractions of the elbow flexors in the 20%–100% range of the maximal voluntary contraction (MVC), estimations were made of the root mean square (rms) and the mean frequency (MF) of the power density spectrum distribution, from the surface electromyogram (EMG) and sound myogram (0SMG) signals, detected at the belly of the biceps brachii muscle. Compared to the young controls, the MVC was lower in the elderly subjects (P < 0.05); at the same %MVC the rms and the MF of EMG and SMG were lower (P < 0.05) in elderly subjects; the rms and MF of the two signals increased as a function of the effort level in all groups. Only in the 80%–100% MVC range did the EMG-MF level off and the SMG-rms decrease; in contrast the young controls, at 80% MVC the high frequency peak in the SMG power spectrum density distribution was not present in the elderly subjects. The results for MVC and %MVC can be related to the reduction in the numbers of muscle fibres in aged subjects. In particular, the lack of fast twitch fibre motor units (MU), attaining high firing rates, might also explain the result at 80% MVC. In 80%–100% MVC range the two signals rms and MF behaviour may have been related to the end of the recruitment of larger MU with high conduction velocity, and to the further increment of MU firing rate in the biceps brachii muscle beyond 80% MVC, respectively. Thus, the coupled analysis of the EMG and SMG with force suggests that in the elderly subjects the reduction of the number of muscle fibres may have co-existed with a MU activation pattern similar to that of the young subjects.     

The relationship between contraction and relaxation during fatiguing isokinetic shoulder flexions. An electromyographic study

Summary Knowledge of the strength, endurance and coordination of the shoulder muscles during dynamic contractions in healthy women would contribute to the understanding of symptoms in that part of the body in patients with myalgia. Twenty clinically healthy women performed single maximal forward shoulder flexions at four different angular velocities (0.57–3.14 rad·s⁻¹). The same subjects also took part in two endurance tests (at angular velocities of 0.57 and 2.09 rad·s⁻¹, respectively) consisting of 150 repeated maximal shoulder flexions. Electromyographic activity (EMG) was registered from four shoulder flexors using surface electrodes. Work was used as the mechanical variable. During the endurance tests subjects rated their perception of fatigue in the shoulder muscles. Work and the amplitude of the EMG signals decreased with angular velocity. The mean power frequency of the EMG was constant in the span of angular velocities investigated. During the endurance tests, work and the mean power frequency decreased during the initial 40–60 contractions followed by stable levels. The relative work level was higher at 2.09 than at 0.57 rad·s⁻¹. Greater relative increases of the signal amplitudes of EMG occurred at 2.09 than at 0.57 rad·s⁻¹. The EMG activity between the flexions (during the supposed passive extension) was higher at 2.09 than at 0.57 rad·s⁻¹. Such a high activity was associated with a low mechanical performance at 2.09 rad·s⁻¹. It is suggested that the initial sharp decreases in work and in mean power frequency reflect the fatiguing of the fast twitch motor units. Dynamic work consisting of continuous activity could predispose to muscle complaints.     

Muscle sound and electromyogram spectrum analysis during exhausting contractions in man

Summary The changes in the soundmyogram (SMG) and electromyogram (EMG) frequency content during exhausting contractions at 20070, 40%, 60070 and 80% of the maximal voluntary contraction (MVC) were investigated by the spectral analysis of the SMG and EMG detected from the biceps brachii muscles of 13 healthy men The root mean squares (rms) of the two signals were also calculated. Throughout contraction the EMG rms always increased while this was true only at 20% MVC for the SMG. A marked decrease was detected at 60% and 80% MVC. With fatigue the EMG spectra presented a compression towards the lower frequencies at all exercise intensities. The SMG showed a more complex behaviour with a transient increase in its frequency content, followed by a continuous compression of the spectra, at 60% and 80% MVC, and a nearly stable frequency content at lower contraction intensities. This study suggested that different aspects of the changes in the motor unit's activation strategy at different levels of exhausting contractions can be monitored by SMG and EMG signals.     

Shoulder muscle load and muscle fatigue among industrial sewing-machine operators

Summary Physiological responses to physical work were assessed for 29 female industrial sewing-machine operators during an 8-h working day under ordinary working conditions. During sewing-machine work, the average (left and right) static load in the trapezius muscle was 9% of the maximal electromyogram (EMG) amplitude (% EMG_max), while the average mean load was 15% EMG_max, and the average peak load was 23% EMG_max. The static load level was unrelated to the muscle strength of the sewing-machine operators, which for the group as a whole was within the normal range. The load levels remained unchanged during the working day, while changes in the EMG mean power frequency and zero crossing frequency rate occurred, both indicating the development of muscle fatigue in left and right trapezius muscle during the working day. In line with this, the rating of perceived exertion in the shoulder and neck region increased during. the working day. Dividing the group of sewing-machine operators into two groups, those with the highest frequency and those with the lowest frequency of shoulder/neck troubles showed that the former group had significantly lower muscle strength, despite the fact that no differences in the surface EMG during sewing were found between the two groups. It was concluded that industrial sewing-machine work involves a pattern of shoulder muscle activity which induces fatiguing processes in the shoulder and neck regions. Furthermore, since the static shoulder muscle load was independent of muscle strength, factors other than working posture may be of significance for the static shoulder muscle load.     

Back and neck extensor loading and back pain provocation in urban bus drivers with and without low back pain

This study assessed low back and trapezius muscle activity in bus drivers, with or without recurrent low back pain (LBP), during the long term driving. In addition, low back and neck–shoulder pain intensities and fatigue were measured and the effect of low back support was observed. Also the possible source of LBP was attempted to assess by vibration pain provocation test and lumbar MRI.

Forty bus drivers (recurrent LBP n = 25) participated in this study. Low back and neck–shoulder pain and subjective fatigue intensity was assessed by visual analogue scales (VAS) before and after driving. Lumbar paraspinal and trapezius muscle activation during driving was measured by surface EMG. Vibration pain provocation test was applied for all subjects.

Average paraspinal myoelectric activity during driving was approximately 1% of MVC in both groups. Average trapezius myoelectric activity during driving was from 2 to 4% of MVC. Trapezius muscle activity was higher in back healthy drivers than in those with LBP. The low back support had no effect either on paraspinal or trapezius EMG activity. Low back and neck–shoulder fatigue increased during driving in both groups especially in those subjects with positive vibration pain provocation. The neck–shoulder pain and fatigue were more severe in drivers suffering from LBP. Low back support had no effect on low back and neck–shoulder subjective fatigue and neck–shoulder pain but tended to limit the LBP increase during driving. Paraspinal muscle loading in urban bus drivers was very limited and either LBP or ergonomic low back support had no effect on it. Trapezius muscle seemed to be less active in drivers suffering from recurrent LBP. Internal disc disruptions may expose to pain and fatigability during driving.     

Muscle oxygenation and glycolysis in females with trapezius myalgia during stress and repetitive work using microdialysis and NIRS

The aim of this investigation was to study female workers active in the labour market for differences between those with trapezius myalgia (MYA) and without (CON) during repetitive pegboard (PEG) and stress (STR) tasks regarding (1) relative muscle load, (2) trapezius muscle blood flow, (3) metabolite accumulation, (4) oxygenation, and (5) pain development. Among 812 female employees (age 30–60 years) at 7 companies with high prevalence of neck/shoulder complaints, clinical examination identified 43 MYA and 19 CON. At rest, during PEG, and STR the trapezius muscle was measured using (1) EMG and MMG, (2) microdialysis, and (3) NIRS. Further, subjective pain ratings were scored (VAS). EMGrms in %MVE (Maximal Voluntary EMG-activity), was significantly higher among MYA than CON during PEG (11.74 ± 9.09 vs. 7.42 ± 5.56%MVE) and STR (5.47 ± 5.00 vs. 3.28 ± 1.94%MVE). MANOVA showed a group and time effect regarding data from the microdialysis: for MYA versus CON group differences demonstrated lower muscle blood flow and higher lactate and pyruvate concentrations. Potassium and glucose only showed time effects. NIRS showed similar initial decreases in oxygenation with PEG in both groups, but only in CON a significant increase back to baseline during PEG. VAS score at rest was highest among MYA and increased during PEG, but not for CON. The results showed significant differences between CON and MYA regarding muscle metabolism at rest and with PEG and STR. Higher relative muscle load during PEG and STR, insufficient muscle blood flow and oxygenation may account for the higher lactate, pyruvate and pain responses among MYA versus CON.     

Effect of step and ramp static contractions on the median frequency of electromyograms of back muscles in humans

The purpose of this study was to compare the electromyogram median frequency (MF) values from two contraction modes (ramp vs step) at different force levels of eight back muscles. A group of 20 healthy male subjects stood in a dynamometer with the trunk in a vertical position and performed trunk extension contractions using the displayed L5/S1 extension moment as visual feedback. The electromyogram (EMG) signals from four pairs of back muscles were collected at 4,096 Hz using active surface electrodes during two 7 s static ramp contractions ranging from 0% to 100% of the maximal voluntary contraction (MVC) and two 5 s static step contractions performed at five forces (10%, 20%, 40%, 60% and 80% MVC). The root mean square (RMS) and MF of the EMG signals corresponding to 250 ms windows were computed at each force level for both contraction modes. The RMS from the ramp contractions were significantly higher than from the step contractions in six muscles. The corresponding MF showed a significant (α=0.05) contraction mode×force interaction in four muscles. A significant contraction mode main effect was obtained in four muscles having higher MF during step than during ramp contractions. These differences were more obvious (10–15 Hz) and more frequent at the lower (10%, 20% and 40% MVC) forces. It was suggested that mechanisms not related to motor unit recruitment might influence MF in contraction modes. These unknown mechanisms contaminate any possible relationship between the MF measurements and muscle composition. Electronic Publication     

The association between nocturnal trapezius muscle activity and shoulder and neck pain

The study investigated the possible association between nocturnal trapezius muscle activity and shoulder and neck pain. Sixty female subjects participated in the study, 33 were classified as pain-afflicted on the basis of shoulder and neck pain reports for the previous 6 months. Electromyographic (EMG) activity was monitored bilaterally from the trapezius in all recordings. EMG recording of the deltoid, biceps, and hand flexors was included for 26 subjects (17 with pain) to provide a comparative basis for evaluation of the trapezius recordings. There was considerable variation in the amount of muscle activity between subjects; however, some subjects presented a continuous, low-level activity pattern throughout the presumed sleep period. Subjects classified as pain-afflicted had significantly higher activity level and longer duration of trapezius EMG activity than the pain-free subjects. The deltoid had significantly more activity than the trapezius, while the biceps and the hand flexors presented similar activity level and duration as the trapezius. Nocturnal trapezius activity was not associated with pain exacerbation the same night. We suggest nocturnal trapezius muscle activity is a pointer to physiological mechanisms that contribute to some forms of shoulder and neck pain, such as trapezius myalgia, but nocturnal muscle activity may not be casually implicated in the pain-induction process.     

Motor unit recruitment and rate coding in response to fatiguing shoulder abductions and subsequent recovery

The purpose of the present study was to investigate motor unit (MU) recruitment and firing rate, and the MU action potential (MUAP) characteristics of the human supraspinatus muscle during prolonged static contraction and subsequent recovery. Eight female subjects sustained a 30° shoulder abduction, requiring 11–12% of maximal voluntary contraction (MVC), for 30 min. At 10 and 30 min into the recovery period, the shoulder abduction was repeated for 1 min. The rating of perceived exertion for the shoulder region increased to “close to exhaustion” during the prolonged contraction, and the surface electromyography (EMG) recorded from the deltoid and trapezius muscles showed signs of local muscle fatigue. From the supraspinatus muscle, a total of 23,830 MU firings from 265 MUs were identified using needle electrodes. Of the identified MUs, 95% were continuously active during the 8-s recordings, indicating a low degree of MU rotation. The mean (range) MU firing rate was 11.2 (5.7–14.5) Hz, indicating the relative force contribution of individual MUs to be larger than the overall mean shoulder muscle load. The average MU firing rate remained stable throughout the prolonged abduction, although firing rate variability increased in response to fatigue. The average concentric MUAP amplitude increased by 38% from the beginning (0–6 min) to the end (24–29 min) of the contraction period, indicating recruitment of larger MUs in response to fatigue. In contrast, after 10 min of recovery the average MU amplitude was smaller than seen initially in the prolonged contraction, but not different after 30 min, while the MU firing rate was higher during both tests. In conclusion, MU recruitment plays a significant role during fatigue, whereas rate coding has a major priority during recovery. Furthermore, a low degree of MU rotation in combination with a high relative load at the MU level may imply a risk of overloading certain MUs during prolonged contractions. Accepted: 6 June 2000